Making a rangefinder cam for the Pacemaker Speed or Crown
Graphic

Brian Oakes

This article applies to the Speed or Crown Graphic with a
top-mounted rangefinder. When you change lenses, you can quite
easily remove the rangefinder cam and replace it with a cam for the
new lens.

It is not difficult to make a cam for another lens. (Focusing
with a telephoto lens/cam combination is not as fast as with the
normal lens- you need to be more careful that the two rangefinder
images are exactly together.)

The cam

If you've never taken the cam out, look carefully at the camera
and the accompanying diagram before acting. It's one of those
things that's easy if you know how, but difficult if you don't.
It's nothing like a Linhof rangefinder cam -- the Graphic cam is
much smaller and in a different place on the camera.

Find the spring-loaded cover under the rangefinder
housing it's between the bellows and the
underside of the rangefinder housing. Hold it open. The camera will
need to be partially upside-down. Rack the focus back and forth and
watch the cam moving right and left in a slot. (It's a piece of
0.8mm thick steel about 25mm long by 10mm wide and, no doubt, easy
to lose.) A follower slides across the surface of the cam, which is
the long side nearest you (that is, nearest the front of the
camera).

Diagram 1 shows roughly what you'll see under the spring-loaded
cover. The arrows show the direction that the components will move
as you focus back towards infinity.

Diagram 1

To remove the cam, rack the focus forwards, past the limit of
rangefinder focusing, which will move the cam to the left. Press
the cam against the spring then slide the cam out towards you.

The factory cam will have the lens focal length stamped on the
under side. Note that the two ends are angled in slightly and there
are two 'feet' on the side opposite the surface. The high (or
left-hand) end of the cam's surface is the position of the follower
when the camera is focused on infinity, and the low end is the
close-focusing position.

To replace the cam, first check that the shaft at the end of the
plunger tube hasn't moved close to the spring shaft. If it has,
tilt the camera to the left to allow the plunger (which is filled
with ball-bearings) to move away, or push the two shafts apart.
Then slide the cam between them and into the slot. It will be held
in place by the spring.

Making another cam

A cam for a longer focal length lens will have a profile with
less of a slope than the one for the normal lens. It will look
something like Diagram 2.

Diagram 2

I used 0.8mm thick brass sheet to make mine. It can be cut with
tin snips (just!). If the sheet metal that you use is too thick,
the cam will wedge in the slot.

If you have some aluminium sheet as well, you could make a
prototype to get the dimensions approximately right before starting
with the brass, which is slower to work with. I suspect that
aluminium would be too soft for the permanent cam.

You can get an idea of the profile of the new cam by looking at
the existing cam. The distance along the surface is proportional to
the distance that the lens panel is racked out, and the height of
the cam affects the movement of the rangefinder mirror.

Although it would be possible to calculate what the profile
should look like, I think it is just as easy to work out the
correct profile by trial and error.

Start by cutting the new cam to the same length as the old, with
the same height (or a little more) at the infinity end and with a
very shallow slope. Use a file and fine emery paper on the surface
of the cam to make it smooth so the follower won't 'grab' when
you're testing the rangefinder.

When you fit a cam for a telephoto lens to the camera you will
probably need to pull the follower towards you to get the right
hand end of the cam into position.

To test the high end of the cam, fit the lens and the cam to the
camera and, using a loupe on the ground glass, carefully focus on
an object that's at a considerable distance. Now check the
rangefinder images. If you need to rack the lens out (that is,
focus closer) to get the images to coincide then you need to remove
more metal from the high end of the cam. If you need to rack back
then you've taken off too much metal. (Either start with a new
piece of metal or, if it's only slightly too small, punch the
left-hand 'foot' of the cam with a hammer and screwdriver to expand
it slightly.) While you're checking the high end of the cam, check
that the low end of the cam is high enough, using the principle
mentioned above.

Keep filing or emery papering until the infinity focus is
correct. Be careful not to take off too much metal - 0.03 mm
makes a noticeable difference. Accurate calipers can help with this
process. You might find it easier to make the final adjustments by
taking a small amount of metal off the 'foot'.

Next, repeat the process at the right-hand end for focusing up
close. When the rangefinder is correct for close and for distant
focusing, try the middle distance. The slope may need to be
slightly concave. The cam for my 127mm lens is concave by about 0.5
mm but the cam for my 270mm isn't.

A cam for a wide-angle lens would, of course, have a steeper
slope than the one for the normal lens.